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A growing number of researchers view age-related neurodegenerative diseases as being due to pathogenic alter egos of otherwise normal proteins. The idea is that the alter egos self-propagate from cell to cell in a steady march through the brain along its anatomic or functional networks. The proteins vary—Aβ and tau in Alzheimer’s, α-synuclein in Parkinson’s, huntingtin in Huntington’s. The driving forces vary—genetic in Huntington’s, genetic or sporadic in Alzheimer’s and Parkinson’s, infectious in rare human cases of variant Creutzfeldt-Jakob disease. At the Chronic Traumatic Encephalopathy conference held 30 September to 1 October 2012 at the Cleveland Clinic Lou Ruvo Center for Brain Health in Las Vegas, Nevada, Stanley Prusiner of University of California, San Francisco, presented data suggesting that brain trauma, too, can cause such a spread of a pathogenic protein—in this case, tau.

Prusiner in 1997 won the Nobel Prize in Physiology or Medicine for his group’s research on prions as a new infectious principle. In the 1990s, a separate line of research into potential self-propagation of amyloid-β began when British scientists reported that injecting brain extracts of AD or Down’s syndrome brains into marmoset brains accelerated the spread of this pathology far beyond the injection site into remote areas of the marmosets' brains (e.g., Baker et al., 1994). This research became prominent in the AD field when Lary Walker and Mathias Jucker adapted and expanded the experimental paradigm in AD mouse models, and soon after Eliezer Masliah’s, Mark Diamond’s, and others' groups showed similar spreading after a single inoculation of α-synuclein and tau. Using anatomical staging, Heiko Braak’s research suggested the spread of disease proteins for Parkinson’s and AD. The movement of the propagating proteins is a slow process. That explains why these diseases, even the most aggressive autosomal-dominant forms, do not express themselves at birth but take decades to develop, Prusiner said.

At the Lou Ruvo Center conference, Prusiner claimed that CTE might be the newest member of these diseases, adding post-traumatic changes in the brain as a new cause. In his talk, Prusiner focused on FTD and CTE as diseases that occupy a new interface between psychiatry and neurology. While those two diseases are distinct, they do share some symptoms. And, according to Prusiner, both are likely to be caused by the self-propagation of tau prions.

Prusiner showed new data generated with an established experimental system using bigenic mice. These mice not only express the P301S FTDP-17 mutation, but also conveniently display tau deposition in the brain with a rise in bioluminescence that can be quantified over time without having to sacrifice the mice for pathology studies (see Tamgüney et al., 2009; Watts et al., 2011). The mice show a spontaneous rise in bioluminescence at 160 to 170 days of age. If the scientists inject extract from cognitively normal 80-year-old people as controls, that number stays the same. However, it was strikingly different when the scientists injected extract from three different human brains into these mice. One was from a patient who died from progressive supranuclear palsy, a pure tauopathy, and one from a person who died with Pick’s disease, the quintessential tauopathy; both were donated by Bruce Miller’s group at the University of California, San Francisco. Finally, the scientists injected extract from an 80-year-old football player with stage 4 CTE donated by Ann McKee at Boston University. Each of these three samples led to a sustained rise in bioluminescence starting between 83 and 91 days. Subsequent histochemistry confirmed widespread tau deposition at that age, Prusiner said. “At 60 days after inoculation, we see the rise in bioluminescence,” Prusiner told the audience.

“Prion-like behavior of the tau protein could turn out to be the way to address progressive degeneration after the end of the trauma,” said Jeffrey Cummings of the Lou Ruvo Center. How does tau move from cell to cell? For his part, Prusiner suggested that aggregating tau inside the neurons represents the pathogenic agent. He believes it polymerizes and travels to the synapse, where it leaves the cell and enters another. McKee noted that CTE pathology suggests that there is significant extracellular spread as well, quite possibly via glial cells.

Other leaders of neurodegenerative research are advising against the use of the word “prion” in the context of Alzheimer’s, Parkinson’s, Huntington’s, and CTE, for that matter, because none of these diseases spreads from animals to animals, animals to humans, or humans to humans. The spread is from cell to cell, strictly within one organism. To avoid the infectious disease connotation of the term “prion,” AD researchers increasingly refer to this phenomenon as "templating" (e.g., Hardy and Revesz, 2012; ARF related news story).

Research with mice is important not only to understand how CTE develops, but also to build better models for therapy development, Prusiner said. Because the bigenic bioluminescence model predictably turns on deposition two months after inoculation, he considers it suitable for evaluating candidate drugs for the disease.

Likewise, Lee Goldstein of Boston University pronounced his group’s mouse model of CTE validated for drug testing. This model is different. It does not model the hypothesized mode of protein propagation as much as reproduce the initial head trauma people suffer in order to recapitulate the cellular and functional phenotype that unravels from there. To Goldstein, tau’s predilection to deposit in the cortical sulcus carried a "whispering of physics," he said in Las Vegas. He teamed up with blast experts to build a shock tube. It delivers to wild-type mice a pressure wave, immediately followed by a blast wind of more than 350 miles per hour, similar to what a soldier would experience from an IED. A previous ARF related news story covered the published findings on this model. At the Lou Ruvo Center conference, Goldstein noted that, since then, he was struck to see the mice reproduce a feature of human CTE he considers important, if not understood. “The pathology engulfing the vasculature in the affected regions is massive, yet right nearby are blood vessels that are totally normal,” Goldstein said.

After the blast itself, the brains of these mice look normal at a macroscopic level. There is no blood, no crush, no rupture injury. But already two weeks after a single blast, the scientists see a variety of hyperphosphorylated tau isoforms and tauopathy. At that point, Goldstein said, whole brain areas appear wiped clean of living cells. In particular, electron microscopy visualizes extraordinary changes to the cytoarchitecture of the blood-brain barrier. “Astrocytic end feet are filled with fluid. They look very sick. The damage to the barrier is pervasive,” Goldstein said.

Further study has shown evidence of extravasation in both directions, compromising the functional integrity of the barrier. “I would not be surprised if we got invasion of T cells or other peripheral cells into the brain,” Goldstein said. For this reason, the BU group currently focuses on detecting CNS biomarkers in the periphery. Previous research has already detected tau and neurofilament light chain as markers of axonopathy in the CSF (see Part 2 of this series); a blood assay for tau was recently published (Randall et al. 2012).

The cellular damage appears to have functional consequences. Their axonal conduction is slow and LTP disrupted. In the Barnes maze, a hippocampal learning paradigm in which the mouse learns to find a dark hole that allows it to escape from a lit, exposed table, blast-exposed mice cannot remember where the hole is.

“Our animal model is ready for drug testing. It is validated for phospho-tauopathy, for diffuse axonopathy, and many other characteristics of CTE. It replicates what we see in humans who are injured on the ball field and in battlefield,” Goldstein said. In other neurodegenerative diseases, researchers are recommending that in-depth characterization of mouse models and subsequent quality controls with large group numbers precede preclinical drug studies (see ARF related news story).—Gabrielle Strobel.